Move _skip function to C extension module for performance

[Copilot]

Implements the first C extension in foamlib by migrating the performance-critical _skip function from Python to C. This function is called extensively during OpenFOAM file parsing to skip whitespace and comments.

Changes

C Extension Module (src/foamlib/_c/skip.c)

• Implements _skip function in C with identical semantics to Python version
• Handles whitespace and both comment types (// and /* */)
• Properly raises FoamFileDecodeError for malformed input

Build System (pyproject.toml)

• Migrated from uv_build to setuptools.build_meta backend to support C extensions
• Configured extension module under [tool.setuptools.ext-modules]
• Package remains buildable with uv build

Integration

• Updated _parser.py to import C implementation
• Added type stub file (_skip.pyi) for type checking
• Added test suite for C extension (tests/test_c_extensions.py)

Usage

The change is transparent to callers - no API changes:

from foamlib._files._parsing._parser import _skip

# Same interface, now backed by C
pos = _skip(b"  /* comment */  data", 0)

This establishes the pattern for migrating additional parser functions to C.

Original prompt

Move the _skip function from the _parser module to a new C extension module, as the first of many functions to be translated to C

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.72%. Comparing base (5fbadb9) to head (eda51f9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #741      +/-   ##
==========================================
+ Coverage   84.67%   84.72%   +0.04%     
==========================================
  Files          27       27              
  Lines        3315     3286      -29     
==========================================
- Hits         2807     2784      -23     
+ Misses        508      502       -6     

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